Method for CMP removal rate compensation

ABSTRACT

A method for polishing a material layer on a semiconductor wafer to a desired target layer thickness. The method includes calculating a compensated removal rate based on the thickness of material to be removed from a material layer on the wafer according to a standard value; the current material removal rate of the CMP apparatus; and the offset thickness, which equals the difference between the thickness of the material layer which would be attained using the current material removal rate and the target thickness for the material layer. The calculated compensated removal rate is then programmed into the controller for the CMP apparatus, which polishes the material layer at the calculated compensated removal rate to achieve the desired target layer thickness for the layer.

FIELD OF THE INVENTION

The present invention relates to apparatus and methods for the polishingof semiconductor wafer substrates in the fabrication of semiconductorintegrated circuits. More particularly, the present invention relates toa new and improved method which is used to compensate for overpolishingand underpolishing of wafers to enhance precision in chemical mechanicalpolishing operations.

BACKGROUND OF THE INVENTION

Apparatus for polishing thin, flat semiconductor wafers are well-knownin the art. Such apparatus normally includes a polishing head whichcarries a membrane for engaging and forcing a semiconductor waferagainst a wetted polishing surface, such as a polishing pad. Either thepad or the polishing head is rotated and oscillates the wafer over thepolishing surface. The polishing head is forced downwardly onto thepolishing surface by a pressurized air system or similar arrangement.The downward force pressing the polishing head against the polishingsurface can be adjusted as desired. The polishing head is typicallymounted on an elongated pivoting carrier arm, which can move thepressure head between several operative positions. In one operativeposition, the carrier arm positions a wafer mounted on the pressure headin contact with the polishing pad. In order to remove the wafer fromcontact with the polishing surface, the carrier arm is first pivotedupwardly to lift the pressure head and wafer from the polishing surface.The carrier arm is then pivoted laterally to move the pressure head andwafer carried by the pressure head to an auxiliary wafer processingstation. The auxiliary processing station may include, for example, astation for cleaning the wafer and/or polishing head, a wafer unloadstation, or a wafer load station.

More recently, chemical-mechanical polishing (CMP) apparatus has beenemployed in combination with a pneumatically actuated polishing head.CMP apparatus is used primarily for polishing the front face or deviceside of a semiconductor wafer during the fabrication of semiconductordevices on the wafer. A wafer is “planarized” or smoothed one or moretimes during a fabrication process in order for the top surface of thewafer to be as flat as possible. A wafer is polished by being placed ona carrier and pressed face down onto a polishing pad covered with aslurry of colloidal silica or alumina in deionized water.

CMP polishing results from a combination of chemical and mechanicaleffects. A possible mechanism for the CMP process involves the formationof a chemically altered layer at the surface of the material beingpolished. The layer is mechanically removed from the underlying bulkmaterial. An altered layer is then regrown on the surface while theprocess is repeated again. For instance, in metal polishing, a metaloxide may be formed and removed separately.

A polishing pad is typically constructed in two layers overlying aplaten with the resilient layer as the outer layer of the pad. Thelayers are typically made of polyurethane and may include a filler forcontrolling the dimensional stability of the layers. The polishing padis usually several times the diameter of a wafer and the wafer is keptoff-center on the pad to prevent polishing a non-planar surface onto thewafer. The wafer is also rotated to prevent polishing a taper into thewafer. Although the axis of rotation of the wafer and the axis ofrotation of the pad are not collinear, the axes must be parallel.

In a CMP head, large variations in the removal rate, or polishing rate,across the whole wafer area are frequently observed. A thicknessvariation across the wafer is therefore produced as a major cause forwafer non-uniformity. In the improved CMP head design, even though apneumatic system for forcing the wafer surface onto a polishing pad isused, the system cannot selectively apply different pressures atdifferent locations on the surface of the wafer. The thicknessdifference between the highest point and the lowest point on the waferis almost 2,000 angstroms, resulting in a standard deviation of 472angstroms, or 6.26%. The removal rates obtained at the edge portions ofthe wafer are substantially higher than the removal rates at or near thecenter of the wafer. The thickness uniformity on the resulting waferafter the CMP process is poor.

Referring to FIG. 1A, a conventional CMP apparatus 50 includes aconditioning head 52, a polishing pad 56, and a slurry delivery arm 54positioned over the polishing pad 56. The conditioning head 52 includesa conditioning disk 68 which is mounted on a conditioning arm 58 whichis extended over the top of the polishing pad 56 for making a sweepingmotion across the entire surface of the polishing pad 56. The slurrydelivery arm 54 is equipped with slurry dispensing nozzles 62 which areused for dispensing a slurry solution on the top surface 60 of thepolishing pad 56. Surface grooves 64 are further provided in the topsurface 60 to facilitate even distribution of the slurry solution and tohelp entrapping undesirable particles that are generated by coagulatedslurry solution or any other foreign particles which have fallen on topof the polishing pad 56 during a polishing process. The surface grooves64, while serving an important function of distributing the slurry, alsopresents a processing problem when the pad surface 60 gradually wearsout after prolonged use.

The conventional conditioning disk 68 may be of several different types.A conventional brazed grid-type conditioning disk is formed by embeddingor encapsulating diamond particles in random spacings with each other inthe surface of a stainless steel substrate. A conventional dia grid-typeconditioning disk is formed by embedding cut diamonds at regularspacings in a nickel film coated onto the surface of a stainless steelsubstrate. The diamonds are typically coated with a diamond-like carbon(DLC) layer.

The CMP apparatus 50 typically further includes a polishing head 70which is mounted on a rotatable shaft 72 above the top surface 60 of thepolishing pad 56. As shown in FIG. 1B, the polishing head 70 holds androtates a wafer 74 against the top surface 60 of the polishing pad 56 topolish the wafer 74. Before production wafers are polished using the CMPapparatus 50, time is typically allotted to warm the polishing pad 56and facilitate flow of polishing slurry from a slurry container (notshown) to the slurry delivery arm 54. This enhances polishing uniformityamong successive wafers polished on the apparatus 50.

The polishing pad 56 is a consumable item used in a semiconductor waferfabrication process. Under normal wafer fabrication conditions, thepolishing pad 56 is replaced after about 12 hours of usage. Polishingpads may be hard, incompressible pads or soft pads. For oxide polishing,hard and stiffer pads are generally used to achieve planarity. Softerpads are generally used in other polishing processes to achieve improveduniformity and smooth surfaces. The hard pads and the soft pads may alsobe combined in an arrangement of stacked pads for customizedapplications.

A problem frequently encountered in the use of polishing pads in oxideplanarization is the rapid deterioration in oxide polishing rates withsuccessive wafers. The cause for the deterioration is known as “padglazing”, wherein the surface of a polishing pad becomes smooth suchthat slurry is no longer held in between the fibers of the pad. Thisphysical phenomenon on the pad surface is not caused by any chemicalreations between the pad and the slurry.

To remedy the pad glazing effect, numerous techniques of padconditioning or scrubbing have been proposed to regenerate and restorethe pad surface and thereby restore the polishing rates of the pad. Thepad conditioning techniques include the use of silicon carbideparticles, diamond emery paper, blade or knife for scraping or scoringthe polishing pad surface. The goal of the conditioning process is toremove polishing debris from the pad surface and re-open pores in thepad by forming micro-scratches in the surface of the pad for improvedpad lifetime. The pad conditioning process can be carried out eitherduring a polishing process, i.e. known as concurrent conditioning, orafter a polishing process. While the pad conditioning process improvesthe consistency and lifetime of a polishing pad, a conventionalconditioning disk is frequently not effective in conditioning a padsurface after repeated usage.

Prior to the CMP operation, each wafer is typically subjected to a CVD(chemical vapor deposition) or other process to sequentially depositmaterial layers thereon. These layers include conductive layers,insulative layers, via layers and IMD (intermetal dielectric) layers,for example. The subsequent CMP operation polishes each layer to thedesired thickness for precise dimensional control of the devicecomponents to be fabricated in the layers. However, in a modernsemiconductor fabrication facility, wafers in different lots arefrequently processed in different CVD chambers, which vary among eachother in the thickness of a given layer that is deposited on a wafer.

The CMP apparatus carries out polishing operations on each waferaccording to a recipe which is programmed into the controller (notshown) for the CMP apparatus. Because the layers on each wafer musttypically be polished to different thicknesses, each layer on the waferhas its own polishing recipe. The polishing recipe includes suchvariables as down pressure and polish time. However, due to variationsin layer thicknesses between wafer lots processed in different CVDchambers, the CMP apparatus, operating according to a given polishingrecipe for each layer, has a tendency to overpolish some layers andunderpolish other layers in a wafer, resulting in layers of imprecisethickness on the wafer. For this reason, for a given layer on each waferof a given lot of wafers, each polishing recipe is programmed with acompensated removal rate to compensate for this polishing imprecisionand facilitate polishing of each layer to a thickness which is asprecise as possible.

The conventional polishing compensation process described above usuallyinvolves the use of a computer server and supporting software thatincludes first and second tables to aid personnel in the selection ofthe correct polishing recipe having the appropriate compensated removalrate for each of the layers on each wafer in a lot. The first tableincludes a sequential listing of the various lots of wafers, each ofwhich is paired with the various layers to be polished on each wafer. Anexample of such a table is shown below as Table I.

TABLE I Product ID (lot) Layer TMA001 VIA1 CMP TMA001 VIA2 CMP TMA001VIA3 CMP TMA001 VIA4 CMP TMA002 VIA1 CMP TMA002 VIA2 CMP TMA002 VIA3 CMPTMA002 VIA4 CMP

The second table displayed on the server includes a sequential listingof high and low limits for the thickness of each layer in Table I to bepolished, paired with the appropriate polishing recipe to obtain atarget layer thickness that lies within the desired range. An example ofsuch a table is shown below as Table II.

TABLE II Lower THK limit Higher THK limit Recipe 22,000 22,200 IMD84.CAS22,200 22,400 IMD86.CAS 22,400 22,600 IMD88.CAS 22,600 22,800 IMD9.CAS22,800 23,000 IMD92.CAS 23,000 23,200 IMD94.CAS 23,200 23,400 IMD96.CAS23,400 23,600 IMD98.CAS

One of the problems which is inherent in the conventional, table-basedmethod of compensating for CMP polishing imprecision is that, inattempts to achieve a layer thickness which is as close as possible tothe target layer thickness, the method is capable of correctiveover-polishing or under-polishing only by increments. As can be seenfrom Table II, each layer on a wafer can be over-polished orunder-polished typically by 200 angstroms to achieve a layer thicknesswhich is as close as possible to the target layer thickness for thelayer. As an example, a normal polishing recipe for a given layer on awafer may result in a layer which is 110 angstroms thicker than thetarget layer thickness. Using the compensation removal rate, the recipe,therefore, operates the CMP apparatus to overpolish the layer and removean additional 200 angstroms from the wafer. The result is a layer whichis 90 angstroms (200–110) thinner than the target layer thickness.

As another example, a normal polishing recipe for a given layer on awafer may result in a layer which is 280 angstroms thinner than thetarget layer thickness. Using the compensation removal rate, thecompensation recipe operates the CMP apparatus to underpolish the layerto leave an additional 200 angstroms on the wafer. The result is a layerwhich is 80 angstroms (280–200) thicker than the target layer thickness.

Another problem inherent in the conventional method is that the softwarerequired for the program occupies an inordinately large space on theserver's hard drive. Accordingly, a new and improved method is needed tocompensate for imprecisions in the CMP polishing of wafers.

It is an object of the present invention is to provide a new andimproved method to compensate for variations in the removal of materialfrom material layers on a wafer during CMP.

Another object of the present invention is to provide a new and improvedmethod for achieving precision in the thickness of layers on a waferusing a CMP operation.

Still another object of the present invention is to provide a new andimproved method for calculating a compensated removal rate for theremoval of material from a layer on a wafer.

Yet another object of the present invention is to provide a method forachieving precise thickness of a material layer on a wafer during CMP.

A still further object of the present invention is to provide a methodwhich can be used to remove material of any needed thickness over acontinuum of thicknesses from a layer on a wafer to compensate forover-polishing or under-polishing of the wafer on a CMP apparatus.

Another object of the present invention is to provide a method for CMPremoval rate compensation which requires a relatively low quantity ofspace on a server and is easy to use and maintain.

SUMMARY OF THE INVENTION

In accordance with these and other objects and advantages, the presentinvention is generally directed to a new and improved method forpolishing a material layer on a semiconductor wafer to a desired targetlayer thickness. The method includes calculating a compensated removalrate based on the thickness of material to be removed from a materiallayer on the wafer according to a standard value; the current materialremoval rate of the CMP apparatus; and the offset thickness, whichequals the difference between the thickness of the material layer whichwould be attained using the current material removal rate and the targetthickness for the material layer. The calculated compensated removalrate is then programmed into the controller for the CMP apparatus, whichpolishes the material layer at the calculated compensated removal rateto achieve the desired target layer thickness for the layer.

In a most preferred embodiment of the invention, the method includescalculating a compensated removal rate according to the followingformula:Compensated RR=(non-compensated thickness/non-compensatedthickness+offset thickness)*current removal ratewhere the “non-compensated thickness” is the thickness of material to beremoved from a material layer on the wafer according to a standardvalue; the “offset thickness” is the difference between a prescribedthickness of the material layer which would be attained using thecurrent material removal rate and a target thickness for the materiallayer; and “current removal rate” is the current material removal rateof the CMP apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which:

FIG. 1A is a top view of a typical conventional CMP apparatus for thechemical mechanical polishing of semiconductor wafer substrates;

FIG. 1B is a side view of a polishing head on the conventional CMPapparatus of FIG. 1A, with a wafer (in section) interposed between thepolishing head and a polishing pad in a CMP operation;

FIG. 2 is a cross-sectional view of a wafer having a material layer tobe polished according to the method of the present invention; and

FIG. 3 is a flow diagram which summarizes a typical sequence of stepsaccording to the method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is generally directed to a new and improved methodfor polishing a material layer on a semiconductor wafer to a desiredtarget layer thickness. The method includes calculating a compensatedremoval rate for a particular material layer to be polished or thinnedon a wafer using a CMP process. The method compensates the CMP polishingrecipe to account for variations in the thickness of a given materiallayer deposited on wafers by different CVD or other processing chambersto obtain the desired target layer thickness of the material layer oneach wafer and thereby exert enhanced dimensional control of devicesfabricated on the wafers. Briefly, the method includes calculating acompensated removal rate using a mathematical equation having variablesfor the non-compensated thickness of material to be removed from thematerial layer on the wafer according to a standard value; the currentmaterial removal rate of the CMP apparatus; and the offset thickness, orthe difference between the thickness of the material layer which wouldbe attained using the current material removal rate of the CMP apparatusand the target thickness for the material layer. The calculatedcompensated removal rate is programmed into the CMP apparatus controllerand the apparatus polishes the material layer at the calculatedcompensated removal rate to attain the desired target layer thicknessfor the layer.

Referring initially to FIG. 2, a wafer 10 according to the method of thepresent invention includes a typically silicon substrate 12. A materiallayer 14 is deposited on the substrate 12 typically using a chemicalvapor deposition (CVD) process, which may be conventional. The materiallayer 14 may be a conductive layer or an insulative layer such as a vialayer for the formation of vias therethrough or an intermetal dielectric(IMD), for example. After it is removed from the CVD or other processchamber (not shown) in which the material layer 14 is deposited on thesubstrate 12, the wafer 10, which includes the material layer 14 and theunderlying substrate 12, as well as any additional material layers (notshown) between the substrate 12 and the material layer 14, has a totalwafer thickness 16. Before fabrication of devices on the wafer 10 canresume, the material layer 14 must be thinned or polished using a CMPapparatus until the wafer 10 has a target wafer thickness 20. At thetarget wafer thickness 20, the material layer 14 has a target layerthickness 26 which is optimal for fabrication of device features in thelayer 14.

The polishing rate of the non-compensated polishing recipe for the CMPapparatus to be used in polishing of the material layer 14 is normallydetermined using a standard value for the total wafer thickness whichmay roughly approximate the total wafer thickness 16 of any given wafer10. In the event that all wafers 10 had a total wafer thickness 16 equalto the standard value, the non-compensated polishing recipe would resultin each of the wafers 10 having the target layer thickness 26 and targetwafer thickness 20. In actual practice, however, due to a variety offactors, the total wafer thickness 16 varies somewhat between wafers 10processed in different CVD or other process chambers, and thus, fromthat standard value for the total wafer thickness. All wafers 10 in thesame lot are processed in the same process chamber and therefore havesubstantially the same total wafer thickness 16. Therefore, a prescribedwafer thickness 18 is based on the standard value for the totalthickness of each wafer 10 and is the thickness of the wafer 10 whichwould be obtained using the non-compensated polishing recipe for thematerial layer 14 on the actual, non-standard wafers 10. Use of thenon-compensated polishing recipe for the layer 14 would result inremoval of a non-compensated thickness 24 from the material layer 14, toobtain the prescribed wafer thickness 18 of each of the wafers 10 in thesame lot. The prescribed wafer thickness 18 is frequently greater thanthe target wafer thickness 20, as shown in FIG. 2. Alternatively, theprescribed wafer thickness 18 may be less than the target waferthickness 20. The difference between the prescribed wafer thickness 18and the target wafer thickness 20 is the offset thickness 22. The totalwafer thickness 16 ranges from typically about 20,000 to about 26,000depending on the number and thickness of the material layers thereon.

Referring again to FIG. 2 and to FIG. 3, according to the method of thepresent invention the non-compensated thickness 24 of material to beremoved from the material layer 14, based on the standard rather thanthe actual value for the total thickness of the wafer 10, is initiallydetermined according to techniques known by those skilled in the art, asindicated in step Si of FIG. 3. Typically, the non-compensated thickness24 and the standard value for the total thickness of the wafer 10 aredetermined using a sample or control wafer on which the material layerto be polished is deposited according to ideal processing conditions.This non-compensated thickness 24 is the value normally called for bythe non-compensated CMP polishing recipe to be removed from the materiallayer 14 and would result in the prescribed wafer thickness 18 of thewafer 10 if the CMP operation were actually carried out according tothat non-compensated polishing recipe, as heretofore noted. Thenon-compensated thickness 24 may range from typically about 500angstroms or lower to about 12,000 angstroms, although this range may bebroader depending on the application. Next, the offset thickness 22 isdetermined by subtracting the target wafer thickness 20 from theprescribed wafer thickness 18, as indicated in step S2 of FIG. 3. Theoffset thickness 22 may range from 1 angstrom to over 1,000 angstroms,depending on the application. Then, as indicated in step S3 of FIG. 3,the current removal rate of the CMP apparatus for the material layer 14is determined typically using a sample wafer (not shown) having thereona material layer the composition of which is identical to that of thematerial layer 14 on the wafer 10. The current removal rate may varyfrom typically about 1,000 angstroms/minute to about 5,000angstroms/minute or higher, depending on the material of the materiallayer 14.

After the non-compensated thickness 24, the offset thickness 22 and thecurrent removal rate of the CMP apparatus for the material layer 14 havebeen determined as heretofore described, these values are substituted,as indicated by process step S4 in FIG. 3, as variables into formula (I)below to obtain the compensated RR (removal rate):Compensated RR=(non-compensated thickness/standard thickness+offsetthickness)*current removal rate   Formula (I)

The compensated removal rate is used by the CMP apparatus as thecompensated polishing recipe to polish the material layer 14 down to thetarget wafer thickness 20. Accordingly, as indicated in step S5, thecompensated RR as calculated by the preceding formula is programmed intothe controller of the CMP apparatus, which operates the CMP apparatusaccording to the compensated polishing recipe to remove the materiallayer 14 until the wafer 10 has the target wafer thickness 20, asindicated in step S6.

The method of the invention will be better understood by considerationof the following example.

EXAMPLE

A lot of wafers was subjected to chemical vapor deposition (CVD) todeposit an intermetal dielectric (IMD) layer thereon. Thenon-compensated thickness of IMD material to be removed from each waferin the lot was 9,000 angstroms. The offset thickness, or differencebetween the target material layer thickness and the prescribed materiallayer thickness, was 200 angstroms. The current removal rate of the CMPapparatus was 3,000 angstroms/min. These values were substituted intothe formula (1) above to obtain a compensated removal rate of 2934.8.This compensated removal rate value was programmed into the controllerof the CMP apparatus, which polished the IMD layer according to theprogrammed compensated removal rate to obtain an IMD layer having thetarget layer thickness.

While the preferred embodiments of the invention have been describedabove, it will be recognized and understood that various modificationscan be made in the invention and the appended claims are intended tocover all such modifications which may fall within the spirit and scopeof the invention.

1. A method of polishing a material layer on a wafer, comprising thesteps of: determining a non-compensated thickness to be removed from thelayer; determining an offset thickness; determining a current removalrate; calculating a compensated removal rate using said non-compensatedthickness, said offset thickness and said current removal rate; andpolishing the layer according to said compensated removal rate.
 2. Themethod of claim 1 wherein said calculating a compensated removal ratecomprises the step of calculating said compensated removal rateaccording to the following Formula:Compensated removal rate=(non-compensated thickness/non-compensatedthickness+offset thickness)*current removal rate.
 3. The method of claim1 wherein said determining an offset thickness comprises the steps ofdetermining a prescribed material layer thickness, determining a targetmaterial layer thickness and determining a difference between saidprescribed material layer thickness and said target wafer thickness. 4.The method of claim 3 wherein said calculating a compensated removalrate comprises the step of calculating said compensated removal rateaccording to the following formula: Compensated removalrate=(non-compensated thickness/non-compensated thickness+offsetthickness)*current removal rate.
 5. The method of claim 1 wherein saiddetermining a current removal rate comprises the step of providing asample wafer, providing a sample layer on said sample wafer, andpolishing said sample layer.
 6. The method of claim 5 wherein saidcalculating a compensated removal rate comprises the step of calculatingsaid compensated removal rate according to the following formula:Compensated removal rate=(non-compensated thickness/non-compensatedthickness+offset thickness)*current removal rate.
 7. The method of claim5 wherein said determining an offset thickness comprises the steps ofdetermining a prescribed material layer thickness, determining a targetmaterial layer thickness and determining a difference between saidprescribed material layer thickness and said target material layerthickness.
 8. The method of claim 7 wherein said calculating acompensated removal rate comprises the step of calculating saidcompensated removal rate according to the following formula: Compensatedremoval rate=(non-compensated thickness/non-compensated thickness+offsetthickness)*current removal rate.
 9. A method of polishing a materiallayer on a wafer, comprising the steps of: determining a non-compensatedthickness to be removed from the layer according to said a standardtotal wafer thickness; determining an offset thickness: determining acurrent removal rate; calculating a compensated removal rate using saidnon-compensated thickness, said offset thickness and said currentremoval rate; and polishing the layer according to said compensatedremoval rate.
 10. The method of claim 9 wherein said calculating acompensated removal rate comprises the step of calculating saidcompensated removal rate according to the following formula: Compensatedremoval rate=(non-compensated thickness/non-compensated thickness+offsetthickness)*current removal rate.
 11. The method of claim 9 wherein saiddetermining an offset thickness comprises the steps of determining aprescribed material layer thickness, determining a target material layerthickness and determining a difference between said prescribed materiallayer thickness and said target material layer thickness.
 12. The methodof claim 11 wherein said calculating a compensated removal ratecomprises the step of calculating said compensated removal rateaccording to the following formula: Compensated removalrate=(non-compensated thickness/non-compensated thickness+offsetthickness)*current removal rate.
 13. The method of claim 9 wherein saiddetermining a current removal rate comprises the step of providing asample wafer, providing a sample layer on said sample wafer, andpolishing said sample layer.
 14. The method of claim 13 wherein saidcalculating a compensated removal rate comprises the step of calculatingsaid compensated removal rate according to the following formula:Compensated removal rate=(non-compensated thickness/non-compensatedthickness+offset thickness)*current removal rate.
 15. The method ofclaim 13 wherein said determining an offset thickness comprises thesteps of determining a prescribed material layer thickness, determininga target material layer thickness and determining a difference betweensaid prescribed material layer thickness and said target material layerthickness.
 16. The method of claim 15 wherein said calculating acompensated removal rate comprises the step of calculating saidcompensated removal rate according to the following formula: Compensatedremoval rate=(non-compensated thickness/non-compensated thickness+offsetthickness)*current removal rate.
 17. A method of programming a CMPapparatus to polish a material layer on a wafer, comprising the stepsof: determining a non-compensated thickness to be removed from thelayer; determining an offset thickness; determining a current removalrate; calculating a compensated removal rate using said non-compensatedthickness, said offset thickness and said current removal rate;programming said compensated removal rate into the CMP apparatus; andpolishing the layer according to said compensated removal rate using theCMP apparatus.
 18. The method of claim 17 wherein said calculating acompensated removal rate comprises the step of calculating saidcompensated removal rate according to the following formula: Compensatedremoval rate=(non-compensated thickness/non-compensated thickness+offsetthickness)*current removal rate.
 19. The method of claim 17 wherein saiddetermining an offset thickness comprises the steps of determining aprescribed material layer thickness, determining a target material layerthickness and determining a difference between said prescribed materiallayer thickness and said target material layer thickness.
 20. The methodof claim 17 wherein said determining a current removal rate comprisesthe step of providing a sample wafer, providing a sample layer on saidsample wafer, and polishing said sample layer.